| The accurate measurement of flame temperature and soot concentration is conducive to in-depth understanding and accurate control of the combustion process.Among the non-contact optical diagnoses of the combustion process,the reconstruction of flame temperature field and soot concentration field utilizing flame image combined with solving algorithms for radiation inverse problem has attracted more and more attention due to its simple device,low cost and reliable measurement accuracy.In this paper,the effects of different factors on the reconstruction of flame temperature field and soot concentration field were studied by using the reconstruction model based on radiation imaging.Then,the calibration of image intensity and absolute radiation intensity was further explored.Finally,the reconstruction of the temperature field and soot concentration field of typical diffusion flames were experimentally studied.The specific work contents and conclusions are as follows:Firstly,the effect of lens optical center uncertainty on the reconstruction of flame temperature and soot concentration of a coaxial diffusion flame cross-section were studied,and the effects of radiation intensity measurement error and ray number were comprehensively considered.The results showed that for the practical CCD camera,the lens optical center uncertainty had little influence on the reconstruction of flame temperature and soot concentration,and the effect of the lens optical center uncertainty had no obvious dependence on the change of the SNR of the measurement system.Therefore,the front edge center of the lens could be used as the lens optical center for practical reconstruction.The increase of radiation intensity measurement error reduced the reconstruction accuracy of flame temperature and soot concentration apparently and had a greater impact on the reconstruction of soot concentration.To ensure the actual reconstruction accuracy,the SNR of the measurement system should be greater than 46 d B.With the increase of ray number,the average relative reconstruction error of flame temperature and soot concentration decreased first and then flattened out,while the maximum relative reconstruction error fluctuated to some extent.Moreover,the increase in ray number had a more obvious improvement effect on the reconstruction accuracy of soot concentration.Then,the influence of coefficient matrix measurement error and edge effect on the reconstruction of flame temperature and soot concentration of a counterflow flame crosssection was studied.The results showed that there was no obvious difference in the reconstruction accuracy of flame temperature between the case of only coefficient matrix measurement error and the case of only radiation intensity measurement error,but the reconstruction accuracy of soot concentration for the case of only coefficient matrix measurement error was significantly lower.The overall reconstruction accuracy decreased and the radiation intensity measurement error was dominant when the measurement error was in both coefficient matrix and radiation intensity.If some flame edges with weak radiation signals were ignored to reconstruct flame temperature and soot concentration,the reconstruction results at flame edges,especially the reconstruction of soot concentration,could have a large reconstruction error.Based on the above results,the image boundary was proposed as the unified reconstruction boundary,and the two-dimensional distribution of flame temperature and soot concentration were reconstructed under different ray numbers,different SNR,different soot concentrations and different reconstruction boundaries for a coaxial diffusion flame.The results showed that the radiation intensity measurement error was the main factor affecting the reconstruction accuracy,but the flame temperature and soot concentration in the sooting area still had reliable reconstruction accuracy when the SNR was as low as 46 d B.The improvement of the reconstruction accuracy of flame temperature and soot concentration in the sooting area became relatively insignificant with the increase in ray number.The low soot concentration in the non-sooting area near the flame center was the main reason for the large reconstruction error in this area.The extension of the reconstructed boundary had little influence on the reconstruction accuracy of the flame temperature and soot concentration in the sooting area,and the change in the reconstruction accuracy had no obvious dependence on the extension of the reconstruction boundary.Finally,the calibration of image intensity and absolute radiation intensity for different cameras and camera parameters was explored,and the reconstruction of the temperature field and soot concentration field of typical diffusion flames was further investigated experimentally.The calibration experiments showed that the R and G of the blackbody furnace target surface image could be adjusted by adjusting the aperture,exposure time,ISO and white balance of the DSLR camera,but the difference between R and G could be reduced by decreasing R and increasing G to some extent only by adjusting the white balance color temperature.Compared with the DSLR camera,the RGB of the blackbody furnace target image taken by the industrial CCD camera had better cooperativity and monotonicity with the calibration temperature and met the linear relationship with the exposure time.Based on this,the realization process of the calibration method of absolute radiation intensity based on the exposure time was illustrated.Based on the calibration results above,the reconstruction model was used to reconstruct the temperature field and soot concentration field of the coaxial diffusion flame.The results showed that the distribution trend of reconstructed temperature and thermocouple measured temperature was consistent,but the former was always higher,and the biggest difference in temperature in the central axis was 10.54%,while the maximum radial temperature difference was 9.32%.The reconstructed results of flame temperature and soot concentration under different fuel flow,oxygen concentration and diluted gas atmosphere could accurately reflect the influence of these factors on flame temperature and soot concentration.The reconstructed model was further used to reconstruct the temperature filed and soot concentration field of the counterflow diffusion flame.The results showed that the radial distribution of temperature was more uniform,while the radial distribution of soot concentration was not uniform near the fuel side,and there were local larger reconstruction values.The flame temperature on the central axis increased,which was consistent with the simulation results,and the soot concentration increased first and then decreased.In practical reconstruction,the lens optical center uncertainty still had little influence on the reconstruction of flame temperature and soot concentration.The DSLR camera was used for the reconstruction of the temperature field and soot concentration field of the counterflow diffusion flame under the magnetic field,the results showed that when the magnetic field was around the fuel nozzle and oxidizer nozzle,the flame temperature increased and soot concentration decreased in both cases.And when the magnetic field was around the oxidizer nozzle,the increase in flame temperature and decrease in soot concentration were more obvious. |
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